1. Field of the Invention
This invention relates to a surface emitting semiconductor laser, a surface emitting semiconductor laser array, and a method for manufacturing a surface emitting semiconductor laser, and more particularly, relates to a surface emitting semiconductor laser, a surface emitting semiconductor laser array, and a method for manufacturing a surface emitting semiconductor laser which are of polarization plane control type used as a light source of optical information processing devices, optical communication devices, and image forming devices using light.
2. Description of the Related Art
While a circular emitting spot is usually required in the laser application technology, a surface emitting semiconductor laser, featured in that the emitting spot is formed easily in a circular shape, has attracted much attention recently because of convenience as a light source which is capable of two-dimensional accumulation. A surface emitting semiconductor laser usually has the isotropic physical configuration in the perpendicular direction to the emitting surface (high axial symmetry), and such configuration leads to no difference in characteristics with respect to two orthogonal axial directions (referred to as two axial directions hereinafter) in the plane horizontal to the emitting surface. As the result, it is known that the polarization plane is directed to the two axial directions in the same probability.
The direction to the two axial directions in the same probability results in impossibility to use polarization-dependent optical elements such as lenses having no special coating and polarization beam splitters as they are, and when many elements are used in the same optical system, the difference of polarization plane between elements is reflected disadvantageously on the light quantity variation, and to avoid such problem various methods have been developed to stabilize the polarization plane in one direction.
Japanese Patent Publication No. 2701596 is an example of such method. In this method, as shown in FIG. 25, the first reflection mirror layer 52 doped with n-type impurity comprising a multi-layer film formed by laminating GaAs layers and AlAs layers alternately, an active layer 70 having an InGaAs layer 56 which is served as a quantum well and provided between two AlGaAs layers 54 and 58, and the second reflection mirror layer 64 doped with p-type impurity comprising a multi-layer film formed by laminating GaAs layers and AlAs layers and a metal electrode layer are formed on a GaAs substrate 50, and the second reflection mirror layer 64 and the active layer 70 are mesa-etched to form a square post 60 served as a vertical resonator structure, and metal films 8a and 8b are provided partially on the mesa side of the post 60 to cause the difference in resonator loss, and thus the light is oscillated in the specified polarization direction and the polarization plane is directed in one direction.
Japanese Published Examined Patent Application No. Hei 7-73139 is another example of such method. In this method, as shown in FIG. 26(b) and FIG. 26(c), an n-type AlAs/GaAs distribution Bragg reflecting mirror (referred to as DBR hereinafter) 52, an n-type Al0.4Ga0.6As layer 54, an In0.2Ga0.8As active layer 56, a p-type Al0.4Ga0.6As layer 58, and a p-type AlAs/GaAs DBR 64 are formed successively on an n-type GaAs substrate 50, and the n-type Al0.4Ga0.6As layer 54, the In0.2Ga0.8As active layer 56, the p-type Al0.4Ga0.6As layer 58, and the p-type AlAs/GaAs DVR 64 are mesa-etched to form a square post 60 served as a vertical resonator structure, and when SiN film 72 is formed on the mesa-sides of the post 60, the temperature is kept at 300xc2x0 C. on one pair of facing sides and at 100xc2x0 C. on another pair of facing sides.
Because the tensile stress exerted on the post 60 is different between the above-mentioned two axial directions when the temperature is cooled down to the room temperature, the stress exerted in the two axial directions in the active layer plane horizontal to the emitting surface is different, one polarization mode out of two axial directions is diminished to stabilize the polarization plane to one direction.
However, because conventional surface emitting semiconductors have the air post structure, if the post diameter is reduced to reduce the threshold current then the optical-output is reduced undesirably, and on the other hand, if the post diameter is increased to obtain desired optical output then the threshold current increases and the horizontal mode becomes unstable un:desirably, so it is difficult to obtain a surface emitting semiconductor laser with reduced threshold current.
In the methods described hereinabove, the metal films 8a and 8b or SiN film 72 are formed on the mesa side which is approximately perpendicular to the semiconductor substrate plane, however, since it is difficult to form an even film reproducibly on a side approximately perpendicular to the semiconductor substrate plane and it requires special high level technique, these methods are not preferable from the viewpoint of reliability for accurate manufacturing of elements.
It is the first object of the present invention to provide a surface emitting semiconductor laser and a surface emitting semiconductor laser array which are capable of controlling the polarization plane of a laser beam to one direction and obtaining a low threshold current. It is the second object of the present invention to provide a method for manufacturing a surface emitting semiconductor laser which is capable of controlling the polarization plane of a laser beam to one direction and enhancing element performance.
To accomplish the above-mentioned first object, A surface emitting semiconductor laser, comprising: a semiconductor substrate; a first reflection mirror layer of the first conduction type formed on the main plane of the semiconductor substrate; an active layer having a quantum well laminated above the first reflection mirror layer; a post having a second reflection mirror layer of the second conduction type different from the first conduction type for constituting an resonator structure together with the first reflection mirror; a plurality of peripheral high resistance layers having a periphery of high resistance and inserted between the first reflection mirror layer and the second reflection mirror layer; and wherein at least two layers of the plurality of peripheral high resistance layers are different in the proportion of non-high resistance therein.
As described above, because plurality of peripheral high resistance layers having the periphery of high resistance inserted between the first reflection mirror layer and the second reflection mirror layer are formed and at least two layers out of the plurality of peripheral high resistance layers are different from each other in proportion of non-high resistance to give the stress different in reflectance distribution and intensity between orthogonal two axial directions, as the result, anisotropy in oscillation threshold gain is caused, only the mode in the direction of smaller threshold gain is obtained selectively, and the polarization plane of the laser beam is fixed in the constant direction.
Because the high resistance area is not conductive, a refractive index waveguide in which a current is narrowed and a beam is confined in the different proportion between the two axial directions orthogonal in the plane parallel to the main plane of the semiconductor substrate is formed, and the element performance of improved low threshold current is obtained.
The proportion of non-high resistance termed in the present invention means the proportion of non-high resistance determined depending on the depth or thickness of the high resistance area and chemical composition of the high resistance area, and includes all the factors that influence the stress exerted on the active layer.
The surface emitting semiconductor laser described above can be comprised at least two layers of the plurality of peripheral high resistance layers are different in the shape of non-high resistance area in the plane parallel to the main plane of the semiconductor substrate.
The surface emitting semiconductor laser described above can be comprised a periphery of high resistance is oxide of the composite of the non-high resistance area.
Because it is known that a layer having a high Al content is easily oxidized by means of high temperature heat treatment in a steam atmosphere and is different in oxidation rate from other layers, only the desired layer is oxidized selectively. Based on the selectivity, the peripheral high resistance layer is formed easily.
The surface emitting semiconductor laser described above can be comprised at least one of the peripheral high resistance layers has the periphery of high resistance which is different in proportion between arbitrary two axial directions orthogonal in the plane parallel to the main plane of the semiconductor substrate.
The layer described hereinabove functions to easily give the stress different in reflectance distribution and intensity between the orthogonal two axial direction to the active layer to cause anisotropy in oscillation threshold gain, only the mode in the direction of the smaller threshold gain is obtained selectively and the polarization of the laser beam is fixed in the constant direction.
The surface emitting semiconductor laser described above can be comprised the shape of the non-high resistance area of at least one of the peripheral high resistance layers in the plane parallel to the main plane of the semiconductor substrate is regular polygonal or regular circular.
The above-mentioned plurality of peripheral high resistance layers are by no means limited as long as at least two layers out of the plurality of peripheral resistance layers are different in proportion of non-high resistance, all layers may be layers having the periphery of high resistance different in proportion between arbitrary orthogonal two axial directions in the plane parallel to the main plane of the semiconductor substrate, or may be combination of a layer having the periphery of high resistance different in proportion between the arbitrary orthogonal two axial directions and a layer having the non-high resistance area in the shape of regular polygon or regular circle in the plane parallel to the main plane of the semiconductor substrate.
The surface emitting semiconductor laser described above can be comprised the shape of the post in the plane parallel to the main plane of the semiconductor substrate is regular polygonal or regular circular.
Because, in the present invention, plurality of peripheral high resistance layers having the periphery of high resistance inserted between the first reflection mirror layer and the second reflection mirror layer are provided and at least two layers out of the plurality of peripheral high resistance layers are different in non-high resistance area to give the stress different in reflectance distribution and intensity between the orthogonal two axial directions to the active layer even if the shape of the post in the plane parallel to the main plane of the semiconductor substrate is regular polygonal or regular circular, as the result, anisotropy in oscillation threshold gain is caused, only the mode in the direction of the smaller threshold gain is obtained selectively, and the polarization plane of the laser beam is fixed in the constant direction.
A surface emitting semiconductor laser array can be obtained by arranging a plurality of the surface emitting semiconductor lasers described above on the same one semiconductor substrate.
Because the polarization plane of the laser emitted from the surface emitting semiconductor laser of the present invention is fixed in the constant direction, the variation of polarization plane between elements is reduced, and the variation in beam quantity on a surface emitting semiconductor laser array is reduced.
To accomplish the above-mentioned second object of the present invention, a method for manufacturing a surface emitting semiconductor laser, comprising: a lamination step for forming a laminate on a main plane of a semiconductor substrate, wherein a first reflection mirror layer of a first conduction type, an active layer having a quantum well, and a second reflection mirror layer of a second conduction type different from the first conduction type, wherein second reflection mirror layer constitutes a resonator structure together with the first reflection mirror, are formed successively, and an insertion layer having an Al content higher than those of other layers of the laminate is formed between the first reflection mirror layer and the second reflection mirror layer during the laminate is being formed; an etching step for forming a protrusion by removing the laminate partially by means of etching until the top surface or side surface of the insertion layer is exposed so that the protrusion has the minor axis in one of the arbitrary two axial directions orthogonal in the plane parallel to the main plane of the semiconductor substrate; a first selective oxidation step for rendering the insertion layer highly resistant by oxidizing the insertion layer from the side-surface intersecting with the minor axis of the protrusion; a post forming step for forming a post by etching the side-surface intersecting the major axis of the protrusion until the top surface or side surface of the insertion layer is exposed; and a second selective oxidation step for oxidizing the insertion layer exposed to the side surface of the post from the side-surface to render the periphery of the insertion layer high resistant.
According to the method described above, a surface emitting semiconductor laser having the peripheral high resistance area having the periphery of high resistance different in proportion between the two axial directions orthogonal in the plane parallel in the main plane of the semiconductor substrate can be manufactured relatively easily through two etching processes and two selective oxidation processes.
To accomplish the above-mentioned second object of the present invention, the other method for manufacturing a surface emitting semiconductor laser, comprising: a lamination step for forming a laminate on a main plane of a semiconductor substrate, wherein a first reflection mirror layer of a first conduction type, an active layer having a quantum well, and a second reflection mirror layer of a second conduction type different from the first conduction type, wherein second reflection mirror layer constitutes a resonator structure together with the first reflection mirror, are formed successively, and a plurality of insertion layers having an Al content higher than those of other layers of the laminate is formed between the first reflection mirror layer and the second reflection mirror layer during the laminate is being formed; an etching step.for forming a protrusion by etching the laminate partially until the top surface or side surface of at least one of the insertion layers is exposed so that the protrusion has the minor axis in one of the arbitrary two axial directions orthogonal in the plane parallel to the main plane of the semiconductor substrate; a first selective oxidation step for oxidizing the exposed insertion layer from the side-surface intersecting with the minor axis of the protrusion to render the exposed insertion layer highly resistant; a post forming step for forming a post by etching the side-surface intersecting the major axis of the protrusion until the top surface or side surface of the rest of insertion layers is exposed; and a second selective oxidation step for oxidizing the insertion layer exposed to the side surface of the post from the side-surface to render the periphery of the plurality of insertion layers high resistant.
According to the method described above, the surface emitting semiconductor laser having the structure in which plurality of peripheral high resistance layers having the periphery of high resistance inserted between the first reflection mirror layer and the second reflection mirror layer are formed and at least two layers out of the plurality of peripheral high resistance layers are different in proportion of non-high resistance as described in claim 1 to claim 6, is easily manufactured through two etching processes and two selective oxidation processes.
The invention described above can be comprised selective oxidation is carried out so that the shape of the non-high resistance area in the plane parallel to the main plane of the semiconductor substrate of at least one the insertion layer is regular polygonal or regular circular or the shape of the post in the plane parallel to the main plane of the semiconductor substrate is formed regular polygonal or regular circular by the post forming step.
The invention described above can be comprised the periphery of the insertion layer is oxidized differently in proportion between the two axial directions orthogonal in the plane parallel to the main plane of the semiconductor substrate by causing the difference in the degree of oxidation between the first selective oxidation step and the second oxidation step.